The present invention relates to a coated cutting tool insert particularly useful for turning of heat resistant superalloys (HRSA) and stainless steels under wet conditions. Very hard fine grained substrates in combination with a (Ti,Al)N PVD-coating greatly improves the notch wear resistance with maintained good flank wear resistance, resistance against plastic deformation and secondary edge notch wear resistance.
Superalloys and stainless steels are developed specifically for applications demanding exceptional mechanical and chemical properties at elevated temperatures. Superalloys are used in aircraft engines while stainless steels are used in a broad range of applications. Almost every metallurgical change made to improve the high temperature properties makes it more difficult to machine these alloys. For stainless steels, this is most prominent for austenitic and so called duplex stainless steels.
As high temperature strength is increased, the alloys become harder and stiffer at the cutting temperature. This results in increased cutting forces and increased wear on the cutting edge during machining.
Because stronger materials generate more heat during chip formation and because the thermal heat conductivity of these alloys are relatively low, very high cutting temperatures are generated. This also contributes to an increased wear of the cutting edge.
Turning can generally be divided in roughing, semi-roughing, semi-finishing and finishing. In roughing and semi-roughing, the depth of cut is generally larger than the nose radius and in HRSA and stainless steels, notch wear on the leading edge and secondary edge are often the critical wear mechanisms together with plastic deformation, flank and crater wear. In semi-finishing, and finishing the depth of cut is generally smaller than the nose radius and the flank wear, plastic deformation and crater wear dominate.